1. Field of the Invention
The invention relates to transmission from a station provided with multiple antennae. The station may be, for example, a base station of a cellular communication system.
2. Description of the Related Art
Multi-antenna transmission and/or reception can be used in a communication system providing wireless communications between stations. Communication systems adapted to provide wireless communication for mobile users are well known. The wireless communication media can be provided between a base station of a communication network and a user equipment. A mobile user equipment is often referenced to by the term mobile station. Wireless communication may also be provided between two user equipment, between two stations of a communication network or between a satellite based station and another station.
A wireless communication system can be used for various types of communication, such as for voice communication or data communication. A wireless system may provide circuit switched or packet switched services or both. In packet switched services data (e.g. speech data, user data, video data or other data) is communicated in data packets. The development in the wireless communication has lead to systems that are capable of transporting data in substantially high data rates i.e. the so called high speed data (HSD).
In a cellular system the mobile user equipment may access the communication network via access entities referred to as cells, hence the name cellular system. One skilled in the art knows the basic operational principles and elements of a cellular network and these are therefore not explained herein in any greater detail. It is sufficient to note that a cell can be defined as an radio access entity that is served by one or several base stations (BS) serving mobile station user equipment (UE) via a wireless interface therebetween. Examples of the cellular networks include networks that are based on access systems such as the CDMA (Code Division Multiple Access), WCDMA (Wide-band CDMA), TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), or SDMA (Space Division Multiple Access) and hybrids thereof.
A mobile user equipment (UE) may communicate simultaneously with a plurality of antenna branches of a base station. FIG. 1 shows an example where a user equipment 1 is receiving signals from two antenna branches 3, 4 of a base station 2. This type of transmission arrangements are sometimes referred to as tx diversity.
The mobile user equipment and the base may transmit to each other data such as control messages, user data and so on. The messages may, for example, be feedback messages. The feedback messages may relate to the tx diversity. The following will discuss a more detailed example that relates to a third generation wideband code division multiple access (3G WCDMA) system.
In WCDMA based systems the above referred high speed data may be enabled e.g. by means of the so called high speed downlink packet access (HSDPA) technology. At the present it is assumed that in the high speed downlink packet access (HSDPA) each user equipment receiving data on a high speed downlink shared channel (HS-DSCH) also has an associated dedicated channel (DCH) allocated. The dedicated channel may be mapped to a dedicated physical channel (DPCH). The DPCH is typically divided into dedicated physical data channel (DPDCH) and dedicated physical control channel (DPCCH) both in the uplink and the downlink. Data such as the power control commands, transport format information, and dedicated pilot symbols are transmitted on the DPCCH. Information such as diversity feedback information may be transmitted on DPCCH in the uplink. It shall be appreciated that although the HSDPA has been specified for use in the WCDMA, similar basic principles may be applied to other access techniques.
Use of the so called closed loop transmit diversity modes have been defined e.g. in the Third Generation Partnership Project (3GPP) technical specification 25.214 version 5.2.0 which was released on Sep. 2002 and titled ‘Group Radio Access Network; Physical layer procedures (FDD) (Release 5)’. A specific reference is made herein to paragraph 7 ‘Closed loop mode transmit diversity’ on pages 43 to 51 of this document wherein two different closed loop modes are proposed. These modes are referenced to in the specification as ‘closed loop Mode 1’ and ‘closed loop Mode 2’.
The general principles of closed loop mode transmit diversity for DPCH transmission can be seen from FIG. 1. Channel coding, interleaving and spreading may be done in a per se known manner, e.g. as in non-diversity mode. As the skilled person is familiar with these operations and they are not essential for the understanding of the invention, they are not described in any greater detail.
In the transmission from the base station 2, a spread complex valued signal is fed to both transmission antenna branches 3 and 4 of the base station 2. The signals are weighted with antenna specific weight factors w1 and w2 at multipliers 7 and 8. The weight factors can be described briefly as being the corresponding phase adjustments in the closed loop mode 1 and phase and amplitude adjustments in closed loop mode 2. These factors are determined by the user equipment 1, and may be signalled to the base station 2 on the uplink DPCCH. Typically the weight factors would be complex valued signals (i.e.,wi=ai+jbi).
For the closed loop mode 1 different orthogonal dedicated pilot symbols are sent on the 2 different antennae 3 and 4. For closed loop mode 2 the same dedicated pilot symbols are sent on both antennae. The transmissions may occur on the DPCCH.
The mobile user equipment 1 may choose from a closed loop mode specific set of transmission weights complex weight factors w1 and w2. The weight factors are typically chosen such that they should maximize the received power at the mobile user equipment. The power can be writtenP=wHHHHw,                 where        
  w  =      [                                        w            1                                                            w            2                                ]  is the weight vector,
  H  =            [                                                  h              1                                                          h              2                                          ]        =          [                                                  h                              1                ,                1                                                                        h                              2                ,                1                                                                          ⋮                                ⋮                                                              h                              1                ,                L                                                                        h                              2                ,                L                                                        ]      is the channel matrix,                hi,j is the jth multipath component of the channel between transmission (tx) antenna iε[1,2] and the mobile user equipment, and L is the number of multipath components.        
The mobile user equipment 1 then feeds back to the base station 2 the selected complex weight factors. This may occur on the uplink (UL) dedicated physical control channel (DPCCH).
At the base station 2 the same complex valued signal is then fed to both transmitting antenna branches 3 and 4. The antennae are then weighted with antenna specific weight factors w1 and w2. One of the transmission antennae is weighted by the so called real weight factor w1. This means that the antenna is not rotated from the phase thereof, i.e. the antenna is not phase adjusted. Such an antenna, which is not phase adjusted, will be called in this specification as the reference antenna.
The other antenna is weighted by the weight factor w2. This antenna is phase adjusted as the phase thereof is rotated. Such a phase adjusted antenna will be referenced to in this specification as the diversity antenna.
Introduction of these modes has proven to improve downlink performance in the multi-antenna arrangements. A reason for the improvement is utilization of the feedback information at the base station. As explained above, the feedback information is determined by the mobile user equipment i.e. the mobile station (MS) based on signals it receives from the base station (BS). Therefore the feedback information should, in theory, reflect accurately the manner the mobile user equipment receives from the base station.
However, use of the feedback makes the closed loop modes sensitive to non-idealities that are likely to occur in real life implementations. For example, both closed loop modes may suffer from non-idealities such as feedback errors, feedback delay, multi-path propagation and so on.
Various antenna verification algorithms may be used to minimize the performance loss caused by feedback errors. In general terms, the antenna verification can be defined as a process by means of which the receiving station may verify that it receives in the requested manner from the transmitting station. One of the basic principles of these verification algorithms is that they try to detect any feedback errors. If an error is detected, an appropriate corrective action may then be taken.
An undetected feedback error may severely degrade the performance of the communication system. As a result of the feedback errors the decoding of the received signals is accomplished by using wrong knowledge of the effective channel at the receiver.
The antenna verification algorithms may utilize dedicated channel estimates. The channel estimates are typically based on dedicated pilot signals transmitted from the base station. In the closed loop mode 1 the dedicated pilots are tx-antenna specific. In closed loop mode 2 the same dedicated pilot is transmitted from both tx-antennas. As the tx-weighting is applied also on the dedicated pilot signals, the dedicated channel estimate(s) contain information regarding the effective channel and the weights used for transmission.
The verification is typically performed based on signals from the diversity antenna.
Antenna verification can be used to mitigate, for example, the following problem. To be able to decode the received signal correctly, the receiver should have the knowledge of the effective channel at the receiver. The dedicated channel estimate contains the information of the used tx-weight. Therefore, if the decoding of the signal is done by using dedicated channel estimates, there is no need to verify the used tx-weights.
The discontinuous DPCH pilot signals are typically transmitted with lower power than continuous primary common pilot channel (CpiCH) signals. Therefore it is considered as more desirable from the system performance point of view to use CpiCH estimates instead of the DPCH estimates. However, when the effective channel is formed by using the CpiCH estimates, information about the used tx-weights should somehow be made available. This is so since the CpiCH signal is not tx-weighted. If the receiving station uses wrong assumption of the effective channel in the decoding (i.e., effective channel is created by using wrong assumption of the used tx-weights), this may result, among other things, bit errors as wrong symbols are detected.
One skilled in the art is familiar with the basic principles of the antenna verification and various possibilities for verifying antennas. For example, a straightforward verification algorithm may be employed wherein a 4-hypothesis test is applied per a slot for the closed loop mode 1. A possibility is to use a simplified beam verification (SBV) that requires only a 2-hypothesis test per slot. For the closed loop mode 2, antenna verification can also be performed, for example using a 16-hypothesis test per slot. As explained above, for closed loop mode 2, the same pilot sequence is transmitted on both antennas for DPCCH. Those interested will find a more detailed description of the above examples from Annex A of the above referenced third generation partnership project (3GPP) specification TS25.214, version 5.2.0 of Sep. 2002.
In the ideal case where the transmission power is equally divided between the antenna branches the choice of the diversity antenna should not have any significant influence to the performance of the closed loop modes. However, non-idealities are likely to be present at the transmitter side. These may cause constant power offset i.e. unbalance between the antenna branches. In such circumstances there may be a need for selection of an appropriate diversity antenna. This may be required especially in order to improve the performance.